Interval running issues/bugs

Hello there. Sorry I am new and this is my first post, so I am not used to how this system works.
Anyway I have had my watch for about 6 months and did my first interval training today.
I set up 5x1km with 4 minutes rest inbetween.
On the fourth km I still had about 250metres to go but it kicked into the 4min rest period. When I finished my run it gave me a new 1km record of 2.45 (which i didn't do) and looking at the map of my route it took me completely off my route for that fourth interval (maybe due to a satellite problem?) and hence why it said I had completed the 1km when i hadn't.
Is this a common issue of tracking runs incorrectly? I had GPS and GLONASS on - is it better just to have GPS on?
Other issues from this run include a maximum cadence of 242 (when on the graph it doesn't go over about 190).
It gave me a record of 2.45 for 1km but looking at my splits it shows 2.55 (another discrepancy).
To be honest I'm not sure if I would buy another garmin. It's nice having the heart rate data after, but with the errors and discrepancies (indoors it is a waste of time), i might switch back to my 10 euro casio. :)

GPS errors happens. With other GPS systems, I used to have GPS wrong data during the first kms due to me starting the activity too early and the system not properly locked. I think it happened to me only a couple of times on my FR 230 in the last 2 years of frequent usage (once the signal is properly locked, it typically works fine). Were you in a bad area (tall buildings/forest)?

Of course if GPS data ar wrong all the records and statistics are wrong.
If you, like me, hate having wrong statistics, an option is to use a service from a third party. You can export your GPX file, create a free account there and you can manually move/erase the wrong GPS data points. Once done you can save the activity and export it (again as GPX) and import it back into Garmin Connect (or any other service you use like Strava). Before doing this I typically delete the original activity with the wrong data.

The "other service" I was mentioning has a name that starts with "R" and ends with "unkeeper".

To me the added value of Garmin (coupled with Garmin Connect) is the flexibility to build complex interval training programs, having the freedom to define the intervals using distance, time, lap-button-press and being able to run my intervals anywhere I want.

GPS errors happens.

Yes, and I've seen some egregious ones.

Is this a common issue of tracking runs incorrectly?

Not really, to my knowledge.

Other issues from this run include a maximum cadence of 242 (when on the graph it doesn't go over about 190).

I've seen erroneous outliers in cadence readings often, but they always show on the graph in Garmin Connect online. If you would like the rest of us to have a look at it, please make the activity record in question visible to everyone and then post its URL.

It gave me a record of 2.45 for 1km but looking at my splits it shows 2.55 (another discrepancy).

Then I'm afraid you misunderstand how personal record detection works on your watch. Your splits during a timed activity has absolutely no relevance or meaning in that regard. If you (hypothetically, say) managed to cover the distance of 1km, from point A that is 3.45km from the start of your next run to point B that is 4.45km from the start, in 2’44” then that will be your new PR for 1K irrespective of where you place the lap markers or how you would prefer your running watch to scan for PRs. I know you said you were running intervals, but that does not mean in reality you stopped immediately upon hitting the end of the 1km interval when you may have been running at your fastest.