Memory thread feedback request (can't find any)

Hi,
Ever since I completed the first three memory threads and killed the incarnations, I've never encountered a memory thread. I am starting to think I am missing a quest checkpoint or something because I feel locked out of this content.
I get that they are probably rare (like the seer?) but I need at least some reassurance from players who have been able to find some because I have found almost nothing on the topic.

a) Is there a step between getting a darker atlas board with petals and finding new memories ?

b) Can they be found only in memories influenced map ?
c) Average rate encounter ?

d) Any other info will be appreciated.

IIRC they are only found in T16 maps and are pretty rare - like every 10+ normal maps.

ive burned full 20 stack of scarabs and found just one thread

I must be really lucky then. I finished a thread of maps then next T16 I had memory lol

Same issue, probably done 20-30 T16's after the "quest" memory strands, not seen a single one... Almost feels like standard league at this point

Ok I've finally encountered a memory tear.

The only thing I did differently than before is I disabled Delirium from my maps. Before than I always had a Delirium Mirror (100% chance).

I honestly doubt that spawning a mirror is preventing the spawn of a memory tear, but I am happy enough that I am going to keep it this way.

If anyone has been able to find a Memory Tear in a map with a Mirror of Delirium, please let me know because I love this mechanic.