Your question concerns a case where Author A publishes original work under the GNU GPL, and then Person B uploads it to GitHub. You ask whether it would be possible for the transaction between Person B and GitHub to affect the ownership of copyright title to the work by Author A.

Simply put: no. Person B does not have standing to perform a transfer of title to GitHub, since they don't hold ownership of the copyright title. Therefore, even if Microsoft drafted an agreement that required Person B to transfer copyright title to GitHub/Microsoft, Person B couldn't fulfill their end of such an agreement, even if they erroneously misrepresented their ability to do so. Person B can't under any circumstances sell or give away a copyright for which they do not hold the title. You might as well also worry that Person B will sell the house you own without owning the deed: Person B doesn't hold any property rights for the property in question, and so does not have standing to transfer those rights.

If Microsoft tried to rely on Person B's grant of copyright title to use Author A's work in other software under terms incompatible with the GPL, Author A would have standing to sue for Microsoft's unauthorized distribution of a derivative work. In such a case, it would be Microsoft's responsibility to ensure they didn't infringe copyright, and their misplaced agreement with Person B does not eliminate their liability for infringing Author A's copyright, especially since the work would be marked with Author A's copyright notices rather than notices with Person B's name. Any agreement with Person B affords no legal defense to Microsoft, so if Microsoft had an interest in violating Author A's copyright, it would be much faster and just as (in)valid to use the work illegally outright, without spurious justifications about some unrelated third party.

Corporations use tools like chains of title and contributor licensing agreements when using outside code (including open source software) to mitigate liability that the outside code might include improperly-licensed parts whose author did not license its use. The idea that a corporation might actively seek out or deliberately engineer such risk within their codebase seems nothing sort of an attempt at legal self-destruction.

Person B does have rights under your GPL license grant, but those rights decidedly do not include the ability to transfer title or even to grant a permissive license to Microsoft to use the software outside the conditions of the GPL. However, those rights do include redistribution rights, so Microsoft was correct to decline your DMCA takedown. Generally, anyone who receives software under the GNU GPL may redistribute it however they please*, and those recipients have the right to distribute it further, etc. This definitely allows recipients to post copies of the software to online servers to facilitate further distribution. You gave permission for your software to be redistributed in this way when you licensed it under the GPL.

The GPL FAQ further clarifies that the GPL does not even require a redistributor to notify you (nor does any FLOSS license require this).

In the case of GitHub in particular, there has been some speculation that their terms of service are technically incompatible with the GPL, but even the FSF itself has found the possibility very unlikely. (See Are the new GitHub Terms of service a kiss of death for open source projects?)

All Microsoft has to do is change their fine print for 1 day and they can claim that my code is theirs.

A much more plausible (but still hardly airtight) formulation of the problem would be if Author A directly uploaded the work to a site that did require transfer of copyright title, as you hypothesize in your "1 day" case. If Author A agrees to be bound by such terms of service, then there is at least a glimmer of plausibility that Microsoft could receive copyright title or a permissive license grant. (I still think there could be serious obstacles to a court upholding such a usage agreement, but it is at least plausible.) As the situation actually is, though, Author A has not uploaded their work to GitHub, and Person B (who is not a rights holder of the work in question) cannot even begin to give Microsoft the necessary rights for this scheme to succeed.

* Redistribution is allowed as long as they meet the requirements to preserve the GPL notices/permissions downstream and that any binaries are accompanied by source; see MadHatter's answer as well.