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-...