Why on my 1099R is only 20K being allowed as a rollover, line B4, when line B2 shows the correct rollover of 30K?

I made a 50K withdrawal from my IRA on 3/1/2018.  On 4/19/2018 I returned 30K to the same IRA.  I was assuming that all of the 30K would be shown on the IRS 1099R as eligibe to rollover.  In effect I am paying tax on 10K that is not in my possesion.