Strange issue with custom 18650 tester
Here's a suggestion about how you could approach your troubleshooting and a hypothesis about the area of the problem.
You said:

While testing the drain circuits (charging circuits were not made at this point), all was perfect

OK - so at that point in time (and perhaps still now), the &quot;drain circuit&quot; for cell 1 behaved normally.

After doing the charging circuits, cells 2, 3, 4 work perfect but cell 1 has the drain circuit on all the time (700mA drain) regardless of whether the mosfet gate is high or low.

So the change happened after you added the charging circuits. Assuming that the channels really are independent, then we can say the problems started after charging circuit 1 was added.
Hypothesis:
I suggest that the problem is not that the intentional drain circuit through the MOSFET is on all the time. You've already said that the discharging occurs when the MOSFET gate is off.
Based on what you said, I think you can only say that cell 1 is being discharged all the time - but my hypothesis is that the discharge path is not via the drain circuit. After all, that was working OK until you added the charging circuit to channel 1.
Instead, I suggest that the charging circuit (e.g. the TP4056 module) on channel 1 has added an unintentional discharge path.
A few troubleshooting approaches which quickly come to mind:

Disconnect the charging circuit from cell 1 (channel 1). Is it still discharging? If not, then the problem isn't in the discharge (drain) circuit, and the discharge path is through the charge components (which is why it stopped when you disconnected them).
or

Swap the charging components between (say) channels 1 and 2. If the unintended discharging moves from channel 1 to channel 2, then the problem is following the charging components. Again, that would confirm the problem is related to the charging components you moved between channels (or perhaps their wiring etc.).
or

Connect an ammeter in series with the MOSFET (not the cell) in channel 1 while the unexpected discharge is happening (i.e. the MOSFET gate is measured as off). I suspect there will be no current through the MOSFET drain-source, again indicating that the discharge path is elsewhere i.e. via the charging components.