Incorrect result when adding back an encrypted input after a multiplication chain

[zooiik]

Summary
What happened: In this simple program below, the plaintext result is stable, but the decrypted FHE result does not match it when I return var11 + x as the final result. If I instead return only var11 (without adding x), the FHE result matches the plaintext result.

What you expected to happen: Correct results whether or not I add back the encrypted input x at the end.

Description
versions affected: 2.11.0

---

minimal POC to trigger the bug:

from concrete import fhe
import numpy as np

@fhe.compiler({"x": "encrypted", "y": "encrypted"})
def test(x, y):
    var2 = y - x
    var3 = (x > var2) * 1
    var4 = 3 + var3
    var7 = -10 * var4
    var8 = var7 - 13
    var9 = var8 * 13
    var11 = 13 * var9
    return var11 + x

x, y = -60, 2
circuit = test.compile(fhe.inputset(fhe.int8, fhe.int8, size=100000))
circuit.keygen()
enc_res = circuit.run(circuit.encrypt(x, y))
result = circuit.decrypt(enc_res)
print(f"Plaintext VS. FHE Computation: {test(x, y)} VS. {result}")
print(circuit)

Output:

Plaintext VS. FHE Computation: -7327 VS. -13762
 %0 = x                         # EncryptedScalar<int8>         ∈ [-128, 127]
 %1 = y                         # EncryptedScalar<int8>         ∈ [-128, 127]
 %2 = subtract(%1, %0)          # EncryptedScalar<int9>         ∈ [-255, 254]
 %3 = greater(%0, %2)           # EncryptedScalar<uint1>        ∈ [0, 1]
 %4 = 1                         # ClearScalar<uint1>            ∈ [1, 1]
 %5 = multiply(%3, %4)          # EncryptedScalar<uint1>        ∈ [0, 1]
 %6 = 3                         # ClearScalar<uint2>            ∈ [3, 3]
 %7 = add(%6, %5)               # EncryptedScalar<uint3>        ∈ [3, 4]
 %8 = -10                       # ClearScalar<int5>             ∈ [-10, -10]
 %9 = multiply(%8, %7)          # EncryptedScalar<int7>         ∈ [-40, -30]
%10 = 13                        # ClearScalar<uint4>            ∈ [13, 13]
%11 = subtract(%9, %10)         # EncryptedScalar<int7>         ∈ [-53, -43]
%12 = 13                        # ClearScalar<uint4>            ∈ [13, 13]
%13 = multiply(%11, %12)        # EncryptedScalar<int11>        ∈ [-689, -559]
%14 = 13                        # ClearScalar<uint4>            ∈ [13, 13]
%15 = multiply(%14, %13)        # EncryptedScalar<int15>        ∈ [-8957, -7267]
%16 = add(%15, %0)              # EncryptedScalar<int15>        ∈ [-9020, -7204]
return %16

If I instead return only var11, the FHE result matches the plaintext result (both are -7267). So this bug only appears when I add back the encrypted input x at the end, which might be related to how the final addition with an encrypted input is handled after a long multiplication chain.