In 1.14.4 and lower versions, 1*1 jungle sapling is able to grow into a maximum 12 height jungle, or a jungle with 12 jungle log blocks. In general small jungle's tree height is from 4 to 12. However, in 1.15-pre1 1*1 jungle sapling is only able to grow into a maximum 10 height tree, no more 11 or 12 tall small jungle
I planted some jungle trees in 1.14.4 and 1.15 pre1, the images below are the result. You can't get a small jungle that is higher than 10 blocks tall in 1.15 pre1

Code Analyze
I'm using the offical mapping and CFR 0.146 decompiling tool
in net.minecraft.world.level.levelgen.feature.stateproviders.SimpleStateProvider line 160, the heightRandA of JUNGLE_TREE_NOVINE_CONFIG is set to 6. It should be set to 8 according to the behaviour before (jungle's height is 4~12) instead of current value 6 (jungle's height is 4~10)