Trainable variables created by tfp.experimental.vi.util.build_trainable_linear_operator_block are lost after the resulting bijector is wrapped in a tfb.Chain

Creating a linear operator via the function tfp.experimental.vi.util.build_trainable_linear_operator_block and then plugging it into tfb.ScaleMatvecLinearOperatorBlock produces a bijector with trainable variables. If this bijector is then put inside a tfb.Chain(), trainable variables are no longer found by the reflection. The same is not true if the linear operator is created manually. My usage of tfp.experimental.vi.util.build_trainable_linear_operator_block is based on the tutorial Variational_Inference_and_Joint_Distributions

import tensorflow as tf
import tensorflow_probability as tfp
tfb = tfp.bijectors
print(tf.__version__)
# 2.18.0
print(tfp.__version__)
# 0.25.0


# Broken example (should print True)
operators = ((tf.linalg.LinearOperatorDiag,),)
block_tril_linop = tfp.experimental.vi.util.build_trainable_linear_operator_block(
    operators, (1,)
)
scale_bijector = tfb.ScaleMatvecLinearOperatorBlock(block_tril_linop)
assert len(scale_bijector.trainable_variables) > 0
c = tfb.Chain([scale_bijector])

print(len(c.trainable_variables) > 0)
# False



# Working example
LO = tf.linalg.LinearOperatorBlockDiag([tf.linalg.LinearOperatorDiag(tf.Variable([1.0]))])
scale_bijector = tfb.ScaleMatvecLinearOperatorBlock(LO)
assert len(scale_bijector.trainable_variables) > 0
c = tfb.Chain([scale_bijector])

print(len(c.trainable_variables) > 0)
# True

Start by reproducing the Python example with TensorFlow 2.18.0 and TensorFlow Probability 0.25.0, focusing on build_trainable_linear_operator_block, ScaleMatvecLinearOperatorBlock, and Chain. Compare the broken and working examples, then verify that the resulting Chain exposes trainable variables and add or update coverage for that behavior.