For me it seems similar issue on my side
First all my Synology´s, now also all my Windows 10 computers back on v1.10.0 and it runs well and smoothly again.
Starting with v1.11.x all peers were green, but some of the devices were at a high percentage level (mostly 95%-99%) and got stuck. And that was random in every time different peer(s) and more or less directories in such peer(s).
I also try to use -reset-deltas (seems not work on Synology´s) but without change. I was only able to fix this by disconnecting the affected peer(s) and reconnecting them.
Only with v1.11.1, android (Syncthing-Fork / AArch64) up to now I have no problem.