Good to hear they're improving. Maybe they were just cold and had wet feet? I found the following graph of peas in a propagator with and without a lid from
vegplotting. You can see how not having a lid and those peas possibly being colder, slowed them down, but almost caught up again at 5 weeks. Perhaps the soil was colder this year or that the brown you're noticing is from another type of pathogen slowing progress, it's hard to say. Sometimes seeds do carry the pathogens with them.
I learned recently from a NASA plants in space talk that plant growth tends to follow an s-curve where the seedlings start in what's called a lag phase where they're trying to establish roots and once they have they then enter an exponential phase, so sometimes we just have to be patient or work out ways to help like warm the soil. Apparently the lag phase for lettuce is about 15 days in perfect conditions, I'm not sure what it is for peas. Otherwise it might be soil density impeding root growth, soil acidity, moisture or nutrients.
Last week I read a study indicating that legumes are some of the first to die off after floods, and also that in the dig versus no dig garden of Charles Dowding in the UK, the broad beans were the only plant to produce more yield in his dug beds. So legumes may love aerated roots for the rhizobia in the nitrogen-fixing nodules to do their thing!