MapServer WMS: Not all features show, depending on how I zoom and pan

Question

Here is how QGIS shows an irrigation network, received by MapServer via WMS and superimposed on OpenStreetMap:

Here is the same thing, but slightly panned to the right:

Several items are obviously missing. In fact there are items missing in the top picture as well. Different items become visible as I zoom and pan differently.

This link provides this tile (which I subsequently show on leaflet):

This is how QGIS shows that area if I manage to zoom and pan in a way that (I think) all information is shown:

What am I doing wrong? I don't see anything in the MapServer logs. MapServer version is 7.6.2 (prepackaged for Debian).

Appendix

Here is parts of the map file:

MAP
    NAME "aira-poros"
    STATUS ON
    SIZE 9040 7731
    EXTENT 103986.57 3850796.18 1007945.14 4623933.00
    UNITS meters
    PROJECTION
        "init=epsg:2100"
    END
    CONFIG MS_ERRORFILE "/var/log/mapserver/extralayers.log"

    # [Omitting some OUTPUTFORMAT sections]

    WEB
        IMAGEPATH "/var/cache/mapserver/"
        IMAGEURL "/mapserver_tmp/"
        METADATA
            "wms_title" "Aira Poros"
            "wms_onlineresource" "https://poros.irmasys.com/cgi-bin/mapserv?map=/opt/aira-poros/aira-poros/mapserver/extralayers.map&"
            "wms_srs" "EPSG:4326 EPSG:2100 EPSG:3857"
            "wms_enable_request" "*"
            "wms_feature_info_mime_type" "text/html"

            "wfs_title" "Aira Poros"
            "wfs_onlineresource" "https://poros.irmasys.com/cgi-bin/mapserv?map=/opt/aira-poros/aira-poros/mapserver/extralayers.map&"
            "wfs_srs" "EPSG:2100 EPSG:4326 EPSG:3857"
            "wfs_enable_request" "*"
        END
    END

    LAYER
        NAME "irrigation-network" 
        TYPE LINE
        CONNECTIONTYPE OGR
        CONNECTION "irrigation-network.gml"
        METADATA
            "wms_title" "Irrigation network"
            "wfs_title" "Irrigation network"
            "gml_include_items" "all"
            "gml_featureid" "id"
        END
        STATUS ON
        PROJECTION
            "init=epsg:2100"
        END
        CLASSITEM 'category'
        CLASS
            EXPRESSION '1'
            STYLE
                COLOR 0 0 255
                OPACITY 50
                WIDTH 6
            END
            LABEL
                COLOR 0 0 255
                ANGLE FOLLOW
                OFFSET 5 99
                MAXSCALEDENOM 10000
                TEXT "[irri_code]"
            END
            LABEL
                COLOR 0 0 255
                ANGLE FOLLOW
                OFFSET -8 99
                MAXSCALEDENOM 10000
                EXPRESSION (NOT('[Diameter]' = ''))
                TEXT "D=[Diameter] mm"
            END
        END

        # [Omitting three more classes]
    END

    # [Omitting four more layers]
END

Answer 1

It looks like either you have an error in your data, or QGIS/MapServer is having problems reprojecting between EPSG:3857 (WMS requests) and EPSG:2100 (WFS request and native format). You can see looking at them together in QGIS using EPSG:2100.

It looks like in the layer metadata (map file) you currently have

"WMS_SRS" "EPSG:2100"

perhaps change this to:

"OWS_SRS" "EPSG:2100 EPSG:3857 EPSG:4326"

Then QGIS will allow you to select say EPSG:2100 for both the WMS/WFS.

Answer 2

As @nmtoken had suspected (see his answer), the problem occurred when MapServer was reprojecting the data from EPSG:2100 (Greek grid—the CRS of the data files) to EPSG:3857.

The problem disappeared when I upgraded MapServer from 7.6.2 to 8.0.0. I didn't want to compile MapServer and I used a version from Debian bullseye backports, which only has 8.0.0, so I don't know if the problem exists in 7.6.4.