I've found this bug, or a similar version, was also present in the 1.13 pre-releases. Basically, normally when a dispenser with a flint and steel would be facing a block directly, like netherrack for example, it would ignite the block on the top. However, as of 20w16a, this no longer seems to work. A dispenser will not ignite a block, unless if it's facing the block from the top with 1 empty space in between, or facing the empty space above the block from the side.

I have attached a screenshot of what setups will work and won't work anymore. It also includes the F3 screen for any necessary information.