Bows previously worked pretty consistently in terms of damage; a fully charged unenchanted bow would do 9-10 damage, power 1 would add 4.5-5 damage, additional levels of power would add 2.25-2.5 damage (on full charge).

Now, they seem to be slightly weaker on average and have much greater damage variance at full charge. The below are some benchmarks I took (I put 32 creepers in individual, 2 block deep holes on the ground and put a fully charged arrow into each, then punched to get the damage the bow did. For each round, I used creepers with health and maxHealth attributes set to what the highest recorded bow damage is - the damage is on the top row, the frequency is the bottom row):

NO POWER:
11   10   9   8   7   6   Avg
4      8    7  10  2   1   8.97

POWER 1:
15   14   13   12   11   10   9   Avg
4      2     3     9     6     4    4  11.78

POWER 3:
20   19   18   17   16   15   14   13   12   Avg
2      3     7     6     3     2     4     3     2   16.31

POWER 5:
25   24   23   22   21   20   19   18   17   16   15   Avg
2      5     2     6     3     3     0     3     2     2     4   20.34

This is a big deviation from how bow damage worked previously, and there is no note in the changelogs (1.9 through 1.13, since I know that bow damage worked consistently in many 1.8 maps I've played) that would account for such a change, so I assume this is a bug. I have no idea how this would be happening, since using spigot, I measured the velocity of arrows at full charge only having slight variance (2.95 to 3.05).

I also don't think that the damage variance can be accounted for by not fully pulling a bow back, because I held the bow at the full charge position (shaky arm animation) for ~2 seconds before releasing, and when testing a custom enchant through spigot that applied bow damage strictly on arrow velocity, I got results consistent with what bow damage used to offer.