I did some searching and it looks like you are right that there isn't anyone credible officially reporting it. However multiple credible news sources do mention welding doors shut as part of opinion pieces, including this one from Ai Weiwei[1] who presumably has more info about the situation and isn't going to simply speak on Western hearsay.

[1] - https://www.theguardian.com/commentisfree/2020/mar/08/china-...

Yes, as I’ve said elsewhere in this thread the strictness of the Chinese lockdown has been exaggerated in the media/Western imagination.

There were literal videos about it at the same time ( a year ago)

I recall seeing various grainy creepy videos (not just about this) supposedly of China posted on Twitter. I started to wonder who was really pushing these videos. I'd have to see a video corroborated by a reasonably trusted news source.

Eg News sources of the BBC had conflicts of interest about reporting on this ( since they have/had a division in China).

What would even be framed in the media when these were really user videos without journalists. Journalists couldn't report live, so i don't think there's something else than opinion pieces.