Heltec E213 and T190 fail to send longer messages

[Fastomat]

My E213 is struggling to send messages longer than roughly 40 characters. It seems similar to the T114 symptoms reported here:

T114 can't send messages longer than 47 chars #4723

The symptoms of my E213:

• Short messages are received instantly
• messages >40 characters take noticeably a lot longer (retransmits?)
• messages >100 characters mostly fail. "Max Retransmits Exceeded".
• Turning down TX power to a poor 6dBm seems to fix it. All messages up to 200 characters get instantly received.

The numbers are not hard limits as it varies somewhat.

I tested with two receiving nodes. Debug logging and SDR prove that the E213 is actually transmitting. Using EU/868 LongFast with tuned antennas.

Full wipes with firmwares from 2.5.20 to 2.6.6 make no difference.

I wonder if I have a bad sample E213, or if this is a bigger issue maybe similar to the T114.
Anyone with a E213?

Update: My Heltec T190 has identical issues, anything over 30-40 characters fails to other nodes that are a few meters away.

[Fastomat]

To narrow it down I did some testing on LongFast, MediumFast and MediumSlow to find a pattern, while reading the debug log ('meshtastic --noproto --serial'). Symptoms start above roughly 700ms of transmit time. For LongFast that's about 40 characters, for MediumFast it's about 100.

The T190 does actually even worse than the E213, threshold for that node is about 600ms of transmit time.
Output power down to 1dBm and transmissions go flawlessly.

I'm leaning towards a hardware issue and not firmware related.