btrfs errors for no apparent reason on RN212

Thanks for that detailed reply. A few points that you raised:

1. The NAS is powered by an APC UPS connected via USB, which triggers NAS shutdown when the battery goes low. I don't expect there to be more than a few abrupt power-loss events in total, and none since maybe a year.
2. The volume is 70-75% full as of now.
3. I am not yet using snapshots, but I know I'll have to use them once I begin regularly backing up.
4. I have not yet set up the maintenance tasks, good point, I will make sure to use it going forward.

Thanks for the "smartctl -x" command, but I don't see anything being flagged, but I found a couple of lines on the Seagate Ironwolf (the other drive is a WD Red CMR) that might be problematic:

SMART Attributes Data Structure revision number: 10
Vendor Specific SMART Attributes with Thresholds:
ID# ATTRIBUTE_NAME FLAGS VALUE WORST THRESH FAIL RAW_VALUE
1 Raw_Read_Error_Rate POSR-- 117 099 006 - 166373216
3 Spin_Up_Time PO---- 096 096 000 - 0
4 Start_Stop_Count -O--CK 100 100 020 - 164
5 Reallocated_Sector_Ct PO--CK 100 100 010 - 0
7 Seek_Error_Rate POSR-- 083 060 030 - 201372729
9 Power_On_Hours -O--CK 074 074 000 - 22879
10 Spin_Retry_Count PO--C- 100 100 097 - 0
12 Power_Cycle_Count -O--CK 100 100 020 - 164
184 End-to-End_Error -O--CK 100 100 099 - 0
187 Reported_Uncorrect -O--CK 100 100 000 - 0
188 Command_Timeout -O--CK 100 100 000 - 0
189 High_Fly_Writes -O-RCK 054 054 000 - 46
190 Airflow_Temperature_Cel -O---K 063 052 045 - 37 (Min/Max 24/37)
191 G-Sense_Error_Rate -O--CK 100 100 000 - 0
192 Power-Off_Retract_Count -O--CK 100 100 000 - 91
193 Load_Cycle_Count -O--CK 100 100 000 - 193
194 Temperature_Celsius -O---K 037 048 000 - 37 (0 20 0 0 0)
197 Current_Pending_Sector -O--C- 100 100 000 - 0
198 Offline_Uncorrectable ----C- 100 100 000 - 0
199 UDMA_CRC_Error_Count -OSRCK 200 200 000 - 0
||||||_ K auto-keep
|||||__ C event count
||||___ R error rate
|||____ S speed/performance
||_____ O updated online
|______ P prefailure warning

 The raw read and seek error rates seem to be high, but the "value" seems to indicate that's just the internal way they're recorded and that they're not an issue. The WD drive did not have any such issues.

Do you recommend a memory test? Because a number of other posts on the internet on btrfs point out that disk corruption is unlikely to cause these issues, as against RAM corruption.