If it’s only the left one this is likely to be an issue with the connections. Retrace your steps. Especially if you took the whole board out. Reseat your keyboard connector, since it is the conduit for fan signals to and from the board. Make sure none of the pins here are bent or damaged, or inadvertently got any solvent on them or other debris (assuming you used something to loosen up the adhesive). Check both the cable side of the connector and the board side. Of particular note would be pins in the middle of each row, the left fan tachometer pin is the 9th one in from the left on the bottom row. If for whatever reason you took the fans out, reseat those connectors too.
If you didn’t take your board out, same steps still apply, but I would take it out and make sure that no solvent got into in any place it wasn’t suppose to. On the other side of that trackpad connector are some components involved in fan speed regulation, so they could have been damaged.
EDIT to include pictures of where the the Q6000 and Q6050 transistors and affiliated components are located, which are responsible board side for fan regulation.