I have this problem with my bike when I put an amount of pressure on my pedals the chain kinda jumps but doesn’t change to a higher or lower gear and it usually happens when I go on 3X7 and 3X6 when I change it to 2X7 it doesn’t happen as usual as 3X7. I have even changed my chain and derailleur but it still happens what could the cause of this be.
Your chain may slipping over the teeth of the cassette sprockets if they are badly worn. This seems the likeliest as the problem occurs when you put increased force on the pedals. Slipping is more likely to happen when on a smaller chainring at the front because the greater leverage is putting more force on the chain. The solution is to replace the cassette.
Another possibility is that you are getting incomplete 'ghost shifting'. The chain is catching the shift ramps on an adjacent sprocket. This should be fixable by indexing the rear derailleur properly.