Bug #64336 is fixed.  So, returning to this...

The following input produces the same output in GNU nroff mode whether the first line has "FB", "-FB", or is commented out.

.WC -FB
.P
This is my paragraph.
.DF
.TS
L.
This is my big table.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
.TE
.DE
.P
This concludes my paper.

But there is a difference in DWB nroff, so this seems like a legit bug.  ".WC FB" is indeed the default behavior, and ".WC -FB" changes the formatting.

$ diff -u floating-keep-no-WC-FB.txt floating-keep-WC-FB.txt && echo SAME
SAME
$ diff -U1 floating-keep-WC-FB.txt floating-keep-WC-minus-FB.txt
--- floating-keep-WC-FB.txt     2024-01-03 04:18:58.396207730 -0600
+++ floating-keep-WC-minus-FB.txt       2024-01-03 04:18:58.400207719 -0600
@@ -7,4 +7,2 @@

-       This is my paragraph.
-
        This is my big table.
@@ -58,2 +56,4 @@
        48
+       49
+       50

@@ -73,4 +73,2 @@

-       49
-       50
        51
@@ -78,2 +76,4 @@

+       This is my paragraph.
+
        This concludes my paper.

It's a neat trick to push material before the float after it.  Is a diversion involved?  Does DWB mm divert all paragraphs by default??

These arguments to the `WC` macro define registers that are never interpolated.  They therefore have no effect.

Their descriptions are hard for me to understand.  The DWB mm manual doesn't leave me much wiser.  It defines this flag in similar terms but tellingly avoids discussing it in its example of the WC macro's usage.

Why would you not have a break, meaning a line break, when popping a floating display?

A display implies a break anyway.  Always.  Everywhere.

What is the deal with this feature?

Can we just drop it?  It's done nothing for ~30 years with no evident complaint anyway.