I have had issues with Spotify offline, but they've been intermittent, and I don't think they were due to SD card issues (I tried 3 different authentic high end, high speed 256GB cards).
The primary issue was that playback would randomly stop for no discernable reason. Thankfully that issue ceased a few weeks back (don't know why for sure, may have been a Spotify update).
More recently I have had issues with the R5 UI slowing down and even stopping responding entirely! There were several occasions where it wouldn't allow playback of ANY of my offline content! Thankfully, just as I was preparing to post a 'disappointed and giving up on R5' rant I checked whether there was any further Spotify updates, and there was both Spotify and Hiby R5 OS updates available, and since installing them my R5 has been back to performing perfectly, with no halts in playback, or UI slowdowns.
This is a huge relief to me, as I absolutely love the R5 (mostly the sound that I can get out of it, but the compact form factor is also great).
I simply formatted the cards in the R5 (two out of 3 were new cards) and I'd advise you do the same. Other than what others have said above I haven't really anything to add re' sd cards.
When (or if) I can afford Tidal again I will move back to them as there was a small but noticable SQ difference to Spotify, but I'm still in awe of how good this DAP is with all my iem's (current favourite is IMR EDP, followed by Tape).
I've converted my entire iem collection to 4.4 for this thing, and I love it! (well, I have yet to add a removable cable to my Denon AH-D2000's, but it's on the cards!)