If it happens once, I will bolus the usual amount of insulin for the meal but treat the low with an appropriate amount of fast acting glucose. If there are some fast acting carbs in the meal, I’ll eat them all first.
If a low mealtime BG is part of a trend, say the last three days, then I’d look at the basal insulin and adjust.
I would also look at any increase in activity, especially exercise, and take that into account.
The biggest risk in treating mealtime lows is over-treating and rebounding high after the meal. If you fear that possibility, I’d make an effort to take a brisk 15-20 minute walk about an hour after the meal insulin dose.