OP - if you have been shielding since the spring - it’s half year+ that you have been mostly stuck together, just the two of you.
And it must be incredibly hard for anybody, even with the best of relationships.
People are not meant to be this isolated and only interact with one other person.
So - I’d say different rules of survival apply in extreme circumstances, and boundaries may need to be adjusted a bit.
If it were me in your place - I’d assume he was just extremely bored and wanted a little bit of human interactions. And solitude, by the sound of it - if he was waking up at night to just get a bit of alone time. (As a person who needs my alone time to recharge I can get that).
Since he hasn’t used the app since July - my guess is that he got it, had a bit of a look around for entertainment purposes and that was that.
Usually MNtters would tell you how this is a deal breaker, and how they won’t stand for it, etc.
But again - none of us has been locked in with just one person for half a year. None of us know what it feels like.
If I were you - I’d not worry about any emotional affairs and him leaving you. People don’t go on tinder to have that - so no one on there is really interested in chatting for months and months and get nowhere. Unless it’s another bored locked in soul who is also seeking just a bit of human contact.
So if he actually went on it and chatted, I’d just think of it as a bit of interactive porn. And maybe in normal times that can be judged more harshly - but at this time - I’d not. He is just trying to survive this crazy time.
Op, you sound like you are quite concerned about your relationship and him leaving because of developing emotional attachment to someone. I don’t think it’s a risk here. However, rather than obsessing and fearing it - why don’t you actually talk to him. Not accuse or confront, but talk like adults. About how hard it is uk this situation, and it what you or him are struggling with, about boundaries and how they can be adjusted in these times.
So that he (Or your) don’t feel the need to hide, and get though these times as a stronger couple.