I had the same problem for quite a while. Although it looks like it tries to catch up, the sync just never gets there. Also had frequent audio drop outs. There was no guaranty that I would have audio when a vid would start playing or for how long there would be audio. And when the audio dropped it was dead throughout the all videos in the app. Returning to the D¡SH UI there was no audio either without rebooting or power cycling the H3. Now I am having audio passthrough issues with my AVR. When I turn the AVR off and want the H3 to passthrough to my TV via ARC, 100% of the time I have to go to the Dolby Audio settings, turn it off, then back on and I get audio.
Strangely enough for the most part the YouTube behavior has stopped and now Netflix crashes upon startup. Prime seems to be OK . . . for now. I now mostly use an AppleTV 4K and the family uses a Firestick 4K and we have no issues with either and those apps. Or we use the onboard TV apps, also with no issues.
My Internet service is Gigabit Fiber ~940/940Mbps read halfway through the LAN using the SpeedTest.net onboard application. The H3 is hardwired to the LAN and gets, as of a check just now, 346.431Mbps. Current FW on both H3s is H310. These issues were happening long before this.