Well I don't have a solution for this, but I think I found the problem and a workaround: open the ArubaOS-CX device in LibreNMS, wait a good ~5 min for LibreNMS polling cron job to occur, click the ports tab, and the port + total traffic graphs reappear like clockwork.
Pcaps of both the 6410 & 6300M traffic to LibreNMS looked sensible; I'm no SNMP expert, but I could see all the SNMPv3 GetBulkRequest & get-response traffic in wireshark, so this is making me lean towards something inside of LibreNMS as the culprit.
After clicking around the GUI for a bit, I noticed the following error message after clicking "show RRD command" on the total traffic graph, which seems to be a really good clue
When I go to the 6300M total traffic graphs that are actually working, I see hundreds of lines of the following in the "show RRD command" section (would post a screenshot, but I'd rather not share my switch FQDNs with the whole internet)
DEF:inB0=/opt/librenms/rrd/(my switch's FQDN)/port-id2943.rrd:INOCTETS:AVERAGE DEF:outB0=/opt/librenms/rrd/(my switch's FQDN)/port-id2943.rrd:OUTOCTETS:AVERAGE CDEF:octets0=inB0,outB0,+ CDEF:inbits0=inB0,8,* CDEF:outbits0=outB0,8,* CDEF:outbits0_neg=outbits0,-1,* CDEF:bits0=inbits0,outbits0,+ VDEF:totinB0=inB0,TOTAL VDEF:totoutB0=outB0,TOTAL VDEF:tot0=octets0,TOTAL AREA:inbits0#CAE853:'1/1/47 In' GPRINT:inbits0:LAST:%6.2lf%sbps GPRINT:inbits0:AVERAGE:%6.2lf%sbps GPRINT:inbits0:MAX:%6.2lf%sbps GPRINT:totinB0:%6.2lf%sB COMMENT:'
' HRULE:999999999999999#CC7CCC:' Out' GPRINT:outbits0:LAST:%6.2lf%sbps GPRINT:outbits0:AVERAGE:%6.2lf%sbps GPRINT:outbits0:MAX:%6.2lf%sbps GPRINT:totoutB0:%6.2lf%sB COMMENT:'
'
I googled "ERROR: string ends after the = sign librenms" and got a very large number of ideas to tshoot this further within my LibreNMS install.
After spending a good 10-15 min on google, I went back to my 6410 total traffic graph which produced the RRD error, and all the data magically appeared! I'm not 100% sure if this is due to my LibreNMS machine being underpowered, or if I need to leave the page open long enough for the LibreNMS poller to update the data, but this feels like a decent repeatable workaround.
There's a good chance we might end up rebuilding LibreNMS on a newer distro soon, so I think I'm going to live with the workaround on these two 6410s for now. Super curious if anyone else runs into 6410 issues with LibreNMS, or if this is just something wacky on my instance. Thanks for the ideas everyone!
Cheers,
Tom