Sorry you're going through this I don't have any advice to offer but would like to share my experiences with you. I'm not saying this is what's happening but want to put it out there.

I used to date a guy who would train my starters on the computer system they would use.
He was very outgoing and friendly, his training sessions would consist of about 10-15 delegates which he would trian for up to 3 months.
The nature of the job meant that a lot of the new staff were university leavers or just young people from school etc.
Part of the training was team building which meant social events which included him and often ended up with a lot of the team continuing on for some drinks etc in the evening. Turns out he was sleeping with a very young delegate, she would text or call him at weekends evenings etc when we were together and he'd ignore the call. He changed her name in his phone to a guy's name then said he was hanging out with this 'guy' cut a long story short we are no longer together!!

My current partner manages a region. This means he is often travelling in his car to and from, meeting different people. Often spending the day (work day) with various colleagues as that's just part of his job.
He colleagues are a mix or male and female young and old etc but that's just part of his job.
Every so often he will have to stay away for a night where they will meet for a drink and just be social. More often than not as he has an early meeting the next day he's back in his room for 9 going to sleep.
He staff and colleagues call him up from about 8am - up to midnight, again that's his job but he answers every call, is happy for me to be present when he's talking to colleagues and is very open.

I trust him 100% as I know it's purely work.
To be honest at first I was a little wobbly about it as he would have female co workers contact him at 11pm but only because something has gone wrong and it's a business call and it was very business. This has happened how do I sort it etc.
I join him where possible for anything social and work related and the people he works with are lovely. I can tell just by the way he handles calls and social events with me that I have nothing to worry about.

He's always made it clear he's their manager and that's it.

If you feel something is off explore that feeling. Only you know your relationship. Tread carefully but ask questions you need answers to.

Sorry for the rambling!! I hope you're ok