Tiny chilli plants - Help!


(Boy, am I having a hard year with the chillies). I've had this problem before, but never ALL of my plants.
Basically, my plants usually would have reached full maturity now. This years plants have undergone the usual thickening of stem, a large amount of leaf growth and are even now flowering.
HOWEVER, the plant itself is scaled down. I'm well acquainted with chilli plants, and these are proportionately the same, but just really really small. The leaves are small, the flowers are tiny and just drop off. It's in a standard compost - I recently potted them on - No issues there. The roots held the mass together. They just super small. Leaves are normal. No pests, no sign of fungal issues. No discolouration.
HELP!
Thanks

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Green house at night, 20s during the day. The stunting started when I had them inside, under artificial led light and at fairly constant temperature.
Started in heated propagator in my g/house, then onto a heat mat, then on the staging.
I've potted them all into 2L pots yesterday as some leaves were yellowing, but they weren't root-bound at all.
I made a note last year that my Marconi peppers were 4ft (the plants not the peppers) on 8th August, Hope this year's get going soon.
No idea why....
Wisdom is not putting it in a fruit salad.
In experience they wont produce true flowers when stunted in this way. Must have needed different soil nutrients/ drainage.
I hope yours also develop!
I guess the cause is environmental so hope now I've re-potted mine they get a wiggle-on
Wisdom is not putting it in a fruit salad.