Problems with undefined item values in properties cards

I had some issues with wrong item value display when using item.State so now I’m using item.displayState for all items shown in properties cards. Problem is here and there, from time to time the items show undefined instead of the value, even though the value is looking right if I check it in the card itself. Rebooting fixes some values but messes others.

Page YAML:

        - component: oh-property-card
            backgroundImage: /static/img/frequency.png
            subtitle: "='Network: ' + items.PM750_Frequency.displayState"

Item definition:

Number:Frequency         PM750_Frequency      "Frequency [%.2f Hz]"                      <energy> (ePM750) ["Frequency"]   { channel="modbus:data:PM750:holdingPoller:Frequency:number" }

It happens with items from various bindings.

Platform information:
    Hardware: i3 3.7GHz/8GB/SSD128GB
    OS: Windows 10 Pro
    Java Runtime Environment: zulu11
    openHAB version: 3.2.0

Explanation here: