In MultiMC, with a Minecraft 1.16.5 instance (no other mods), and any Forge for 1.16.5 installed, launching the instance offline with a changed player name (via the MultiMC player name box that pops up) causes Minecraft to freeze when I later try to "save and quit to title" after playing. If I uninstall Forge from that instance then saving works as normal without freezing.
Also, launching a MultiMC Minecraft instance offline with a changed player name and with Forge installed doesn't show the changed player name in chat (it shows my main account name), whereas if I uninstall Forge and launch the instance offline with a changed player name, the changed player name does show in chat (which is how it has always worked).
This doesn't happen in an older Minecraft instance, for example in an instance of 1.7.10 with Forge I can change the name and it shows the changed name in chat, and the game also saves without freezing.
I don't know whether this is an issue with Forge or MultiMC but since MultiMC seems to work as expected without Forge installed I thought I would post the issue here first.