Low Fuel Pressure Indication

Just returned from a 1 hr flight - no low fuel pressure warning - looking good!

I will do a few more hours, before making a statement that its probably been the fuel pressure sensor plug seal causing the low pressure indication on TO/Climb out.

I would argue that this is Not a Rotax, Dynon, or Garmin issue.
The sensor is made by Kavlico and it is a Kavlico issue.
That said, Kavlico does not market the sensors for experimental aircraft use.
They are industrial sensors used primarily in HVAC and other stationary equipment.
If it takes an hour for the reference pressure to stabilize; that is not an issue.
The barometric pressure doesn't change that fast except in a hurricane eye.
Even in automotive applications, unless you are climbing Pikes Peak, it will be unusual for your elevation to change much faster than 1000 ft per hour, and certainly never 1000 feet per minute.

When you took on adapting the sensor for experimental aircraft use, You took on the responsibility to confirm that it operated under all flight conditions.

An easy way to resolve the pressure equalization issue would be to fabricate a 2-cent paper washer to place at the sealing point.
It will keep out the dirt and all but the most aggressive water spray, and break the pressure seal to allow the gauge to equalize to the ambient pressure within seconds.

OR drill a 1 mm hole in the underside of the plug female casing (sensor side).

OR slightly damage/slot the seal so that it more easily allows some air movement.

As for apportioning responsibility - seems to me the businesses offering aircraft applications (Dynon, Garmin, etc) that provide these plugs as part of their system, should have, in the first instance,  determined that they were fit for use. 

The plugs themselves are what I would consider to be "top of the line" automotive - so no issues with quality.

That it seems that the information, regarding problems with atmospheric normalising,  has been known, but not commonly so, for a long time, is a failure of the system suppliers. 

On my aircraft the male portion of the connector measures about 2mm shorter than the depth of the female socket. Today I drilled a 2mm hole at the base of the female side diagonally opposite the internal sensor as I didn’t want to damage that with the drill bit, de-burred the hole, blew out the detritus, reinstalled the seal on the male portion and reconnected the whole thing. Took it for a test flight and all is well, just a brief reduction in indicated pressure to 2.9 psi when the aux pump was switched off but almost immediate recovery to 4.5 psi with no fluctuations at any time. I’ll probably still swap to the newer sensor later as I have one at hand but would be comfortable to continue with this sensor as now is knowing that if an indication problem resurfaces, the first thing I would do is give the connector a good clean.