OK, this one is also weird, I've compiled mesa-18.3.0 in my home directory,
exported these two variables:

export LD_LIBRARY_PATH=/home/diego/mesa/lib
export LIBGL_DRIVERS_PATH=/home/diego/mesa/lib/gallium

I startx after that, glxinfo reports the right mesa version (18.3.0) and lsof
says this:

$ sudo lsof | grep radeonsi_dri.so
[sudo] password for diego: 
Xorg      8082                  diego  mem       REG               0,22
84191168     322546 /home/diego/mesa/lib/gallium/radeonsi_dri.so
Xorg      8082 8083 Xorg:cs0    diego  mem       REG               0,22
84191168     322546 /home/diego/mesa/lib/gallium/radeonsi_dri.so
Xorg      8082 8084 Xorg:disk   diego  mem       REG               0,22
84191168     322546 /home/diego/mesa/lib/gallium/radeonsi_dri.so
Xorg      8082 8085 Xorg:sh0    diego  mem       REG               0,22
84191168     322546 /home/diego/mesa/lib/gallium/radeonsi_dri.so
Xorg      8082 8086 Xorg:sh1    diego  mem       REG               0,22
84191168     322546 /home/diego/mesa/lib/gallium/radeonsi_dri.so
Xorg      8082 8087 Xorg:sh2    diego  mem       REG               0,22
84191168     322546 /home/diego/mesa/lib/gallium/radeonsi_dri.so
Xorg      8082 8088 Xorg:sh3    diego  mem       REG               0,22
84191168     322546 /home/diego/mesa/lib/gallium/radeonsi_dri.so
Xorg      8082 8089 Xorg:sh4    diego  mem       REG               0,22
84191168     322546 /home/diego/mesa/lib/gallium/radeonsi_dri.so
Xorg      8082 8090 Xorg:sh5    diego  mem       REG               0,22
84191168     322546 /home/diego/mesa/lib/gallium/radeonsi_dri.so
Xorg      8082 8091 Xorg:shlo   diego  mem       REG               0,22
84191168     322546 /home/diego/mesa/lib/gallium/radeonsi_dri.so
Xorg      8082 8092 Xorg:shlo   diego  mem       REG               0,22
84191168     322546 /home/diego/mesa/lib/gallium/radeonsi_dri.so
Xorg      8082 8093 Xorg:shlo   diego  mem       REG               0,22
84191168     322546 /home/diego/mesa/lib/gallium/radeonsi_dri.so
Xorg      8082 8094 Xorg:shlo   diego  mem       REG               0,22
84191168     322546 /home/diego/mesa/lib/gallium/radeonsi_dri.so
Xorg      8082 8095 Xorg:gdrv   diego  mem       REG               0,22
84191168     322546 /home/diego/mesa/lib/gallium/radeonsi_dri.so

But then I cannot reproduce the bug anymore.

Any ideas?