Apple Watch Series 4 stops recording workout midway through!

I have had my apple series 4 since October 2018, and up until the last few months not really many issues, but something that is really starting to get annoying is the fact midway through a run the watch will end the workout and I have to stop then start a new workout. It has done it a few times in the last few weeks, and for those that are runners you can imagine the pain it causes. I also have an outstanding case open where the HRM will not pick up the heart rate at the beginning of a workout, but to counteract this I have started wearing a separate chest strap and connecting through Bluetooth even though I now I shouldn't have to.

Would love some answers in particular the workout stopping midway through.

I have tried a a number of resets only thing is completely un-pairing and resetting as a new apple watch, of which I really think is a last restore but willing to give it a go if people have had success with this?

Any ideas?