I don't know about underpitching.. but I see a thing or two I wouldn't do myself.
Mainly its that you are perhaps combining a really quite big re-pitch, with an undersized starter (for the amount of yeast)
The object of a starter, is to grow up the correct number of healthy yeast cells to pitch - and dependent on your technique and/or to be pitching them while they are in an active growth phase. The way I see your technique.... you are actually doing neither.
Lets talk about a 1.050 ale 20ish litres - all figures are kinda rough...
Your 300ml of yeast slurry is somewhere between 33% & 600% more cells than the recommended pitching rate for 20L of 1.050 ale... so in my books, you are overpitching somewhere between moderately and massively... and thats just if you re-pitch the slurry without a starter.
If you put that amount of yeast into a starter of 1.5L volume... whats going to happen?? well, there is far and away more yeast than is required to ferment that amount of wort ... so basically the yeast isn't going to do any growing, its just going to ramp straight into fermentation mode. Which means that when you drop it into your beer... it has to stop fermenting, re-evaluate its surroundings and re-set itself to growth mode , basically backtracking and starting again. (I am anthropomorphising ruthlessly here). So really all your 1.5L starter is doing.. is to make the yeast burn up a small amount of resources before it hits your wort... and then you are overpitching anyway.
My suggestion would be to pick a method... either a significant re-pitch, or a starter. Use the calculator that Newguy linked to, to work out how much slurry you should be using for a straight re-pitch ---- OR ----- take a small amount of yeast slurry, about 100billion cells worth (so 120ml of very thin slurrry or 25ml of thick yeast sludge) and pitch that into a starter of the size recommended on the same site.
Not gospel... lots of brewers do starters and pitching in lots of different ways and make it work. You say you aren't generally having problems?? so there really isn't any need to change. But... if you do start to encounter issues, or if you just want to try a different method to see if it makes a difference, either positive or negative. Those are my thoughts.
TB