The ideal germination temperatures for the veg you mention are around
21-27C, so you're dealing with both too hot and too cold. How does the
temperature in the propagator get to 40C in the daytime without the
heat on? Is your greenhouse that warm? It doesn't have an automatic
vent? I don't know whether heat or cold is more detrimental. The cold
will retard germination. Heat may be less injurious.

Well, I was surprised it got that hot too. This was on a fairly sunny day
with the lid on the propagator and the vents open. When I put my hand over
the vent, I could feel the heat coming out. The greenhouse temp was around
25C at this time. When the sun goes in for a while the greenhouse temp is
around 12-18C during the day. Should I bring all my seed trays into the
house, or should I wait a few weeks and see what happens?

My salad leaves have all germinated, but nothing from the "exotic" tray as
yet.