Constraining a part to fixed parts doesn't work

Maybe a bit cryptic title, but I want to build a frame with boxsections and add an angleiron in the corners. These boxsections are assembled at a specific location so they are in the right place. Now I want to add another part to the assembly and constrain it to the boxsections. To stop the boxsections from moving, I added a `Fixed` constrain to them. But the angleiron never gets the right place.

My code below:
```Python
import cadquery as cq

boxsection_w = 20
boxsection_h = 60
boxsection_t = 2
boxsection = (cq.Sketch()
.rect(boxsection_w, boxsection_h, tag='outer')
.rect(boxsection_w - (2 * boxsection_t), boxsection_h - (2 * boxsection_t), mode='s', tag='inner')
.vertices(tag='inner')
.fillet(boxsection_t)
.reset()
.vertices(tag='outer')
.fillet(boxsection_t * 2)
.reset()
)

angleiron_w = 30
angleiron_h = 60
angleiron_t = 5
angleiron = (cq.Sketch()
.segment((0,0), (angleiron_w, 0))
.segment((angleiron_w, angleiron_t))
.segment((angleiron_t, angleiron_t))
.segment((angleiron_t, angleiron_h))
.segment((0, angleiron_h))
.close()
.assemble()
.vertices()[2:5]
.fillet(angleiron_t)
)

frame_w = 700
frame_l = 2400

frame_profile_length = 2200
frame_profile_width = 600

angleiron_lengte = 300

tableprofile_l = (cq.Workplane('XY')
.placeSketch(boxsection)
.extrude(frame_profile_length)
)
tableprofile_l.faces(">X").tag("TL_IN")

tableprofile_w = (cq.Workplane('XY')
.placeSketch(boxsection)
.extrude(frame_profile_width)
)
tableprofile_w.faces(">X").tag("TL_IN")

tabelcorner = (cq.Workplane('XY')
.placeSketch(angleiron)
.extrude(angleiron_lengte)
.copyWorkplane(cq.Workplane('XZ'))
.sketch()
.segment((0,0),(angleiron_w,0))
.segment((0, angleiron_w))
.close()
.assemble()
.finalize()
.cutThruAll()
.moveTo(0, angleiron_lengte)
.sketch()
.segment((0,0),(angleiron_w,0))
.segment((0, -angleiron_w))
.close()
.assemble()
.finalize()
.cutThruAll()
)
tabelcorner.faces("Z").tag("TH_OUT")

tabelframe = (cq.Assembly()
.add(tableprofile_l, loc=cq.Location(-(frame_w - boxsection_w) / 2, -boxsection_h / 2, -frame_profile_length / 2), name="R")
.add(tableprofile_w, loc=cq.Location((-(frame_profile_width) / 2, -boxsection_h / 2, (frame_l - boxsection_w) / 2), (0,90,0)), name="D")
.add(tabelcorner, name="TH1")
)
# add the constraints
(
tabelframe
.constrain('R?TL_IN', 'Fixed')
.constrain('D?TL_IN', 'Fixed')
.constrain('TH1?TH_IN', 'R?TL_IN', 'Plane')
.constrain('TH1?TH_OUT', 'D?TL_IN', 'Plane')
)

tabelframe.solve()

show_object(tabelframe)
```
I have tried to swap the strings of the last two constrains but it doesn't work.