plpgsql variable assignment with union is broken

hi,

i updated our ci pipeline to the latest 14-devel build from the postgresql apt repository:

PostgreSQL 14devel (Debian 14~~devel~20210105.1140-1~285.gitbc43b7c.pgdg110+1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 10.2.1-3) 10.2.1 20201224, 64-bit

i found, that the behaviour of variable assignment in combination with union is not working anymore:

DO $$
DECLARE t bool;
begin
t := a FROM ( SELECT true WHERE false ) t(a) UNION SELECT true AS a;
END $$;

before it worked with pg13 and 14-devel with this build:

PostgreSQL 14devel (Debian 14~~devel~20201126.0540-1~210.gitf3a8f73.pgdg110+1) on x86_64-pc-linux-gnu, compiled by gcc (Debian 10.2.0-16) 10.2.0, 64-bit

is this an intended change or is it a bug?

with kind regards,
richard