I want to say that this warning is just a liability thing and it’s really probably fine, I probably am getting the right amount of oxygen even if it is a little uncomfortable — but my workplace just gave me some new tasks that involve cutting and lifting decently heavy objects, so I want to make sure that it isn’t like genuinely dangerous to wear a mask while I’m doing these things.
I would think it’s a liability thing for sure. If you have an spo2 meter you could also check how you do with it? No permanent harm feels likely as we should adapt to respiratory resistance just fine.
Assuming you are wearing a n95 type mask of dome kind, maybe this study could give you some peace of mind? It states that there are changes in prolonged use, but those are likely to resolve with adaptation and none are outside the normal range.
Anecdotally I’ve personally done weightlifting in a FFP2 mask, it was uncomfortable at first, but I got used to it.
Edit. Here is a systemic review looking spesifically at exercise. I would think that this can be compared to harder labor.