[BUG] If a line is discretized by one element, import and export with meshio does not yield a file readable by GMSH

[TimS-HSU]

I am currently facing an issue with reading in a msh file from GMSH (4.11.1), and saving it again (without modification) to a GMSH msh file. For this, I use the following:

import meshio
msh = meshio.read("test.msh")
meshio.gmsh.write("test2.msh", msh, binary=False)

meshio.__version__ returns 5.0.0.

The initial msh file test.msh can be opened by GMSH again. The resulting msh file test2.msh however cannot. The test.geo file (from which the test.msh is generated) is:

Point(1) = {0, 0, 0, 1};
//+
Point(2) = {1., 0, 0, 1};
//+
Point(3) = {1., 1., 0, 1};
//+
Point(4) = {0, 1., 0, 1};
//+
Line(1) = {1, 2};
//+
Line(2) = {2, 3};
//+
Line(3) = {3, 4};
//+
Line(4) = {4, 1};
//+
Curve Loop(1) = {1, 2, 3, 4};
//+
Plane Surface(1) = {1};
//+
Physical Curve("bottom", 1) = {1};
//+
Physical Curve("right", 2) = {2};
//+
Physical Curve("top", 3) = {3};
//+
Physical Curve("left", 4) = {4};
//+
Physical Surface("domain", 10) = {1};

The resulting test.msh looks as follows:

$MeshFormat
4.1 0 8
$EndMeshFormat
$PhysicalNames
5
1 1 "bottom"
1 2 "right"
1 3 "top"
1 4 "left"
2 10 "domain"
$EndPhysicalNames
$Entities
4 4 1 0
1 0 0 0 0 
2 1 0 0 0 
3 1 1 0 0 
4 0 1 0 0 
1 0 0 0 1 0 0 1 1 2 1 -2 
2 1 0 0 1 1 0 1 2 2 2 -3 
3 0 1 0 1 1 0 1 3 2 3 -4 
4 0 0 0 0 1 0 1 4 2 4 -1 
1 0 0 0 1 1 0 1 10 4 1 2 3 4 
$EndEntities
$Nodes
9 5 1 5
0 1 0 1
1
0 0 0
0 2 0 1
2
1 0 0
0 3 0 1
3
1 1 0
0 4 0 1
4
0 1 0
1 1 0 0
1 2 0 0
1 3 0 0
1 4 0 0
2 1 0 1
5
0.5 0.5 0
$EndNodes
$Elements
5 8 1 8
1 1 1 1
1 1 2 
1 2 1 1
2 2 3 
1 3 1 1
3 3 4 
1 4 1 1
4 4 1 
2 1 2 4
5 1 2 5 
6 4 1 5 
7 2 3 5 
8 3 4 5 
$EndElements

After reading in and saving the file as msh with meshio again, this yields

$MeshFormat
4.1 0 8
$EndMeshFormat
$PhysicalNames
5
1 1 "bottom"
1 2 "right"
1 3 "top"
1 4 "left"
2 10 "domain"
$EndPhysicalNames
$Entities
4 0 1 0
1 0 0 0 0 
2 0 0 0 0 
3 0 0 0 0 
4 0 0 0 0 
1 0 0 0 0 0 0 1 10 4 1 2 3 4 
$EndEntities
$Nodes
5 5 1 5
0 1 0 1
1
0.0000000000000000e+00 0.0000000000000000e+00 0.0000000000000000e+00
0 2 0 1
2
1.0000000000000000e+00 0.0000000000000000e+00 0.0000000000000000e+00
0 3 0 1
3
1.0000000000000000e+00 1.0000000000000000e+00 0.0000000000000000e+00
0 4 0 1
4
0.0000000000000000e+00 1.0000000000000000e+00 0.0000000000000000e+00
2 1 0 1
5
5.0000000000000000e-01 5.0000000000000000e-01 0.0000000000000000e+00
$EndNodes
$Elements
5 8 1 8
1 1 1 1
1 1 2
1 2 1 1
2 2 3
1 3 1 1
3 3 4
1 4 1 1
4 4 1
2 1 2 4
5 1 2 5
6 4 1 5
7 2 3 5
8 3 4 5
$EndElements

I have realized that 1. the entities representing the lines are missing, and 2. the "nodes"(?) representing the elements (previously 1 1 0 0, 1 2 0 0, 1 3 0 0 and 1 4 0 0) are missing (and the entries in the first line are therefore also different). If adding the missing entries in "Nodes" by hand, the lines are discretized, but GMSH reads it in fine. If also adding the entities, no discretization is necessary and GMSH reads it in fine aswell.

Can someone maybe point me to a solution to this problem? Of course my real mesh is a little more complicated, but it boils down to this issue.