Custom layer with Concat afterwards causes an error during QAT modeling

Prior to filing: check that this should be a bug instead of a feature request. Everything supported, including the compatible versions of TensorFlow, is listed in the overview page of each technique. For example, the overview page of quantization-aware training is here. An issue for anything not supported should be a feature request.

Describe the bug
I am building a model that returns feature embedding as an output.
I used MobileNetV3Largs as a baseline with include_top=False option.

after the baseline, I have few layers and concat them at the end of the model.

When I tried to apply QAT on the model though, it returns the error saying:

TypeError: 'str' object is not callable

System information

TensorFlow version (installed from source or binary):
2.15.0

TensorFlow Model Optimization version (installed from source or binary):
0.7.5

Python version:
3.11.7

Describe the expected behavior
QAT model generated well.

Describe the current behavior

TypeError: 'str' object is not callable

Code to reproduce the issue
Provide a reproducible code that is the bare minimum necessary to generate the
problem.

def build_model(args, include_preprocessing=False):
    IMG_SHAPE = (args.input_dim, args.input_dim, 3)
    # Transfer learning model with MobileNetV3
    base_model = tf.keras.applications.MobileNetV3Large(
        input_shape=IMG_SHAPE,
        include_top=False,
        weights='imagenet',
        minimalistic=True,
        include_preprocessing=include_preprocessing
    )
    # Freeze the pre-trained model weights
    base_model.trainable = False
    cl1 = CustomLayer()(base_model.output)
    cl1 = tf.keras.layers.Dropout(0.2, name="dropout_cl1")(cl1)
    
    cl2 = CustomLayer()(base_model.output)
    cl2 = tf.keras.layers.Dropout(0.2, name="dropout_gd2")(gd2)
    
    cl3 = CustomLayer()(base_model.output)
    cl3 = tf.keras.layers.Dropout(0.2, name="dropout_cl1")(cl3)

    concat_cls = tf.keras.layers.Concatenate()([cl1, cl2, cl3])

    x = tf.keras.layers.Dense(512, activation='swish')(concat_cls) # No activation on final dense layer
    model = tf.keras.Model(base_model.input, x)
    
    return model

# This is initial training loop before QAT --> this returns few epoch trained model
model = build_model(args)
model = initial_training(args, model)

def apply_quantization_to_dense(layer):
    if isinstance(layer, tf.keras.layers.Dense):
        return tfmot.quantization.keras.quantize_annotate_layer(layer)
    return layer

annotated_model = tf.keras.models.clone_model(
    model,
    clone_function=apply_quantization_to_dense,
)

quant_aware_model = tfmot.quantization.keras.quantize_apply(annotated_model)

Screenshots
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Additional context
Add any other context about the problem here.

No repository file or test is named. Start with the supplied build_model and apply_quantization_to_dense reproducer, then run quantize_apply on the annotated model to isolate the failure around CustomLayer, Concatenate, and Dense; done means the provided model generates a QAT model without the TypeError.