Definitely, rich soil can be too much for seedling, this is a common thing in all agriculture frankly,... As for Fuxfarms floor scrapings "soil", they are notorious for a variety of quality issues which run the gamut because of their dubious outsourcing and greed... Undercooked soil, bugs, badly off pH (I have repeatedly measured bags with low 5's pH, instant FUBAR), rock/sticks and other garbage debris...
A decade ago they made great soil, that's how they made their bones,... that sourcing is long long gone now, just hype and former good rep' demand driving them to pump their shit out as fast and cheap as they can get away with,... A local supplier where I live actually stopped carrying their soils because of so many come-back problems traced to their soil!
---Fuxfarms home base is only 400 miles away from us, BTW....
That said, some folks do get OK bags, again that's down to the local sourcing and subcontractors making it,...
Usually, it's not the richness that gets the seedling in OF soil, it's the root fragg'ing, nute locking acidic pH....
Also in play is cultivar to cultivar tolerances for such things... Avoid the worry by starting them in milder soil. In a final pot, just make a pocket of about 1L in volume and fill with milder soil, plant into that.... I nest a cup in the pot when filling it up, then remove it....makes a perfect shaped core to plant into