I ran into this with Visual Studio 2019 (on Windows 10). I'm not sure quite how it started, but I do use ReSharper. Closing and restarting VS2019 did not work. I also installed a minor update (probably before restarting VS; I'm not sure if this might have actually locked-in the always-on-top behavior or made no difference--probably the latter, but I mention it just in case). I tried the various solutions here and in other linked pages and none seemed to help (I might not have hit all of them; some were more involved or meant for older versions or otherwise hard to try).
Finally, on a whim, I tried launching a second instance of VS 2019--since they can't both be on top at the same time--and after minimizing the first instance to get to the second, I found that the new instance did not force itself on top of other windows. I closed out the first instance and then re-opened my solution in the new instance, and it no longer forces itself on top. I just checked that it stays fixed after restarting VS 2019 also.
Update: This can help turn off the always-on-top state at a particular time, but it doesn't prevent it from potentially happening again. I'm not sure what causes it, but VS does get back into being always-on-top from time to time when a new VS instance is started. So, it's a temporary but not permanent fix.