Don't know, tbh, I've seen it in Japan and kind of figured it might exist elsewhere too. Since OP provided no further context, I thought I'd mention it.
Ever since I saw it in Japan I realized that it's just the best way. It eliminates all confusion about what "1am Tuesday" means. And 27:59 rolling into 4:00, or 28:59 rolling into 5:00, makes perfect sense since 4-5am is a normal sunrise time most of the year in Japan
I would think a possibility is that it's from a dataset where the data wasn't recorded properly so they are unsure as to whether it was am or pm but didn't want to throw out the data point.
It doesn't. It means AM but it's not the bus driver's bedtime yet so it's not really morning. But your misunderstanding is valid because this is a goofy way of communicating this information.