My 2¢ .....
Most of your issues are from your hand .... user errors.
What soil is that you are using ?
Ph of water used and type
Lights
Feed ( Nutes )
Grow space
Overwatering, over handling , plus fim / topping an already stressed plant will give you exactly what you are seeing. Now you expressed cutting the roots off and treat it like a clone ?
It’s ALREADY rooted ....
User errors are from too much micromanaging and hovering over growth.
Try a bagged mix
PLant seedling ... bury the stem to stop overstretch
Light watering to BASE of stem .......... Not full container.
Allow seedling to SEEK moisture ( tap root will shoot to bottom ) and growth will accelerate
Cotyledon leaves carry basic nutrition inside itself to START SEEDLING GROWTH
Good bagged soil needs NOTHING ... NO EXTRA FEED , NOTHING
And will carry initial growth for weeks on WATER ONLY.
After awhile ( weeks ) soil will start to exhaust ... by then you should be either getting close to flip or veg longer if desired. If growing a longer veg stage then you will supplement VEG NUTES as it needs nitrogen for green growth.
If flipping .... a simple 1/4 to 1/2 strength VEG nute feed to boost it thru transition as it changes gear to flower. The next 2 weeks she will stretch and branch ( harden up ) PRIOR to full flower.
Once it slows or stops .... THEN you can begin BLOOM NUTES.
PH ALWAYS .... WHETHER STRAIGHT PLAIN WATER OR FOR FEED MIXING.
PH 6.3 to 6.5 ( soil ) ...... you can LOCKOUT nitrogen under 6.0 in soil.
Forget test plants .... you are just wasting plants and time by not really trying to correct from start.
Think about it ...... 39 days were just wasted up to a point NOW where you still want to cut on it.
That plant would have been so much more right now if you just grew simply.