The problem could be due to the possibility that there is different value of reversal potential for chloride ions, as pipette solution contains different concentrations of chloride ions.

Do you check the osmolarity of both ACSF and intrapipette solution? Do they differ significantly?

The magnitude of volume-sensitive chloride channels existing in neuron could be one of the problems created.

yes, both osmolarity are fine, ACSF is 315 and Intra is 295, @Marek Brodzki

this is a good point, different cell types probably express chloride transporters and channels @Sheng-Nan Wu

Try to backfill your tip with a low chloride solution. Lower the pipette fast and do not apply pressure to the pipette until you are near the surface of your slice and cell.

Thanks Paula Parra. That's a good option indeed !