I have shared location on maps and it only reports back sometimes, like you might see that the last reported location is 20 minutes old or something. Also I don't think they only use GPS these days, I think its that, but also local wireless networks, and location data based on your daily habits (i.e. you go to work every day, so if we see your work wifi, you're probably at work). All of this put together doesn't really kill battery life anymore.
Any idea if this is because the phone was not able to get the precise location to share? I have done this too but it was on a road trip and I didn't get good coverage at many points.
Assuming you aren't going hiking in some remote place where you could end up somewhere with shitty sky visibility (for GNSS satellites), it's pretty hard not to get a decent position. Even cheap GNSS receivers can get like 1.2m RMS positioning with just 1 band.
I can really only speculate, but my running theory (and I must emphasize, I am really pulling this out of my ass), is that the location updating stuff doesn't trigger while your phone isn't being used (i.e. you leave your phone in your bag all day, it doesn't do much location updating, but if you are browsing Hexbear on it all day, it updates more frequently).
The reason I think this is because I've noticed on days I've been really busy at work, my shared location doesn't seem to get updated as much (not that it matters given I'm in the same place). Honestly power settings and if apps can run in the background seems to be very inconsistent across different phones and I swear it doesn't work 90% of the time so I might just be seeing patterns that don't exist.