You’ve got me puzzled. . .
“Theoretically”, that should NOT happen - even if there’s NO SD card installed, something should be blinking, if for no other reason than to tell you it can’t find the SD card!
Let me get this straight. . .
You have a SD card, we’ll call it SD card “A”.
If you flash Raspbian on it, it works fine and the green LED flashes as it normally should.
If you flash DexterOS on it, it does nothing and the green activity light does not flash at all at any time.
Am I correct?
What kind (brand, size) of SD card is it? Where did you buy it?
If you buy SD cards from a reputable store with a reputation to uphold, chances are you’ll get better products than from some sidewalk vendor in a flea-market somewhere.
The ONLY time I’ve had boot-up strangeness like this is when I’ve used sub-standard SSD cards by strange manufacturers.
Have you tried this experiment with excellent brand cards, like SanDisk, Samsung or Toshiba (Brands like PNY and Kingston simply re-brand other people’s cards, so you don’t really know what you’re getting)
If you live in the U.S. and are near a Micro Center, their store brand SD cards are also quite good. (Their flash drives are also good.)
I bought some cards branded “Smart Buy” and “Perfeo” while in Europe, and they were, (shall we say), “strange”.
I don’t know if this is anywhere near your problem, but good quality SD cards are an absolute must. Nowadays I only buy top-end SanDisk Extreme cards for my Pi - They’re more expensive, but it saves me so many problems and so much grief in the long run.
How old are these cards? Have they been used a lot? Were they working before but suddenly started having problems?
You might have some cards, especially cheaper cards, that have begun to fail.
Let me know what’s happening.