I have noticed the same behavior with my sequences which are having DateTime tags set with timezone adjustment (e.g. UTC+2).
I’ve did some tests and can confirm the this is probably a viewer bug.
Sequence/ photo time shown in the app is taking DateTime tag and ADDING your timezone offset from local windows setting.
So, assuming you have 10:00:00 in GPS tag (UTC), and 12:00:00 in DateTime (UTC+2) and you local PC timezone is the same (UTC+2) the time shown will be DateTime + local timezone offset, or 14:00:00 in this specific example.
You can try it yourself - just change your local windows setting to another timezone, reload a page and you will see the date is changed accordingly
I guess this was done by design but I believe the logic should be changed, instead of reading DateTime tag, mapillary viewer should read GPS DateTime tag which is always in UTC.
In that case time in the viewer will be always shown in local timezone of the PC correctly.
If the logic is to keep time shown matching the realtime of the photo, e.g. 10:00:00 irrespective whether it being opened in London or in Tokyo - than probably image DateTime tag should be parsed without any adjustments from mapillary viewer. The downside of this approach is that DateTime EXIF is not always matching actual time of the place where photo was taken. I personally often adjust all my DateTime tags to UTC before processing and uploading.