"The KPC-3 is notorious for this."
The KPC3 is fine. It's the KPC-3 Plus units that have KISS mode issues.
I had several KPC3 (non-plus) TNCs in service in KISS mode for years. One ran a 2m APRS digipeater for nearly a decade without any problems, it just ran and ran...till a lightning strike got it LOL. Good little units that deal well with the proliferation of poorly modulated signals (hum, overdriven, etc) that 144.3900 is infested with in many areas.
Best I can tell those KPC3+ KISS mode issues were never resolved by Kantronics. Some kludges were proposed and sometimes helped (delay the onset etc) but never fully addressed the issue.
Basically you should never ever use a KPC-3 Plus in KISS mode on APRS. May seem fine for a few minutes, hours or days, but sooner or later that delayed received packet issue will show up.
Alternatives are many and not limited to:
KPC3 (non-plus) turn up often on Fleabay.
The KPC-9612 (plus and non-plus) units don't exhibit the problem, but probably overkill for just 2m APRS use.
AEA/Timewave PK-96 TNCs work well but can be a bit finicky with their KISS modes/firmware.
The SCS Tracker DSP TNC has a kiss mode and can be ran in 5 different modes (HF packet, Robust Packet, 1200 baud, 9600 baud, and 19200 baud) or as a stand-alone (no PC/tablet required) tracker on those modes.
Could always drop a KISS mode bug.
73
Jeff
WA4ZKO
Jeff - WA4ZKO