Alarm snooze function not working at times & at other times the time intervals are all different?

having issues this last week with the preloaded alarm function, a few mornings it hasnt gone off, then snooze hasnt kicked in, this morning my snooze was going off at random time intervals from 10-13 mins instead of previous 9mins. This is happening to my husbands phone, we have carried out the manual reset, switchetd on & off. Deleted the alarms & reset seemed to have got the snooze function to work this morning but the times in between were so random it through out his leaving the house time by nearly 10mins. But he had also noticed at work this week he seemed to have gone over his allocated breaks ( lorry driver) he sets his phone alarm to go off 13mins before end of his break so he knows to finish up what he is eating etc & when the snooze goes off he gets back into his seat ready to start up the engine as it’s all digital & records everything. But he has been going over by a few minutes every time & we couldn’t work out why till this morning. Anyone have any suggestions???