Keyboard closes directly after opening in an extended FormField - flutter

Question

When building a custom form field by extending FormField (with a TextFormField centered within two IconButtons), I encounter unexpected issues with the keyboard.
After a state change due to a button press, the TextFormField is only updated when it has been provided with a key during construction (e.g. UniqueKey()). However, when using keys in this way, the keyboard directly closes after opening. The keyboard only works properly when no key is provided to TextFormField. When the key from the FormField baseclass constructor is provided, the keyboard also doesn't work and TextFormField is not updated when an IconButton is pressed.
When using the iOS simulator, no error is received when the keyboard closes directly after opening. When using a physical device, I get the following error:
Successfully load keyboard extensions
[lifecycle] [u 4EF1D37A-3BC9-4488-BC5A-A32FFFD4094F:m (null)] [com.google.keyboard.KeyboardExtension(2.3.19)] RB query for the extension process state failed with error: Error Domain=RBSServiceErrorDomain Code=1 "Client not entitled" UserInfo={RBSEntitlement=com.apple.runningboard.process-state, NSLocalizedFailureReason=Client not entitled, RBSPermanent=false}
The problem occurs at various versions of flutter, among which is version 3.7.0-1.3.pre.
Is this a bug or should the keys be used in another way?
Code that illustrates the problem
class CustomNumberFormField extends FormField<double> {
final int scale;
final TextStyle? textStyle;
CustomNumberFormField({
required FormFieldSetter<double> onSaved,
this.scale = 0,
this.textStyle,
super.validator,
super.key,
super.initialValue,
}) : super(
onSaved: onSaved,
builder: (FormFieldState<double> state) {
return Column(
mainAxisAlignment: MainAxisAlignment.center,
crossAxisAlignment: CrossAxisAlignment.end,
children: [
Row(
mainAxisSize: MainAxisSize.min,
children: <Widget>[
IconButton(
icon: const Icon(Icons.remove),
onPressed: () {
state.didChange((state.value != null)
? state.value! - 1
: initialValue);
},
),
SizedBox(
width: 100,
child: TextFormField(
///////////////////////////
// Problem occurs here:
key: UniqueKey(), // key: key,
// Keyboard cannot be used when the above keys are used (i.e. UniqueKey() or from baseclass).
// However, state changes due to pressing IconButton do not update text value without using UniqueKey().
// Using the baseclass key also results in closing the keyboard and IconButtons do not update TextFormField
initialValue: NumberFormat("#.#").format(state.value),
keyboardType: TextInputType.numberWithOptions(
decimal: (scale > 0)),
textAlign: TextAlign.center,
inputFormatters: <TextInputFormatter>[
FilteringTextInputFormatter.allow(RegExp(
scale > 0 ? r'[0-9]+[,.]{0,1}[0-9]*' : r'[0-9]')),
TextInputFormatter.withFunction(
(oldValue, newValue) => newValue.copyWith(
text: newValue.text.replaceAll('.', ','),
),
),
],
style: textStyle,
autovalidateMode: AutovalidateMode.onUserInteraction,
onSaved: (value) {
if (value != null && value != "") {
NumberFormat().parse(value).toDouble();
}
},
),
),
IconButton(
icon: const Icon(Icons.add),
onPressed: () {
state.didChange((state.value != null)
? state.value! + 1
: initialValue);
},
),
],
),
state.hasError
? Text(
state.errorText ?? "",
style: const TextStyle(color: Colors.red),
)
: Container()
],
);
},
);
}