BnT refreshing Rend is almost always what I've done right as Rend is about to fade. It's been more annoying to do lately since 4.3 due to Rend no longer applying/refreshing on targets that Thunderclap misses though, but that's just the nature of RNG.
I did post a bug report thread after 4.3 dropped regarding BnT's new bug, but after two minor patches it hasn't been fixed (or it's "intended" and they've said nothing). It really feels like Blizzard is in apathy mode right now.
EDIT: For those interested about the Blood and Thunder bug, I've got some logs to display what it was doing compared to how it's acting now. It normally doesn't create problems, but we all know RNG will do otherwise.
Tier 11 Example (Source:
World of Logs - Real Time Raid Analysis)
[21:41:50.543] Thecheat casts Thunder Clap
[21:41:50.543] Thecheat Thunder Clap Vile Swill Miss
[21:41:50.693] Vile Swill's Thunder Clap is refreshed by Thecheat
[21:41:50.693] Vile Swill's Thunder Clap is refreshed by Thecheat
[21:41:50.693] Vile Swill's Thunder Clap is refreshed by Thecheat
[21:41:50.693] Vile Swill's Thunder Clap is refreshed by Thecheat
[21:41:50.929] Thecheat Thunder Clap Vile Swill 5428
[21:41:50.929] Thecheat Thunder Clap Vile Swill 5291
[21:41:50.929] Thecheat Thunder Clap Vile Swill 5291
[21:41:50.929] Thecheat Thunder Clap Vile Swill 5359
[21:41:51.122] Vile Swill's Rend is refreshed by Thecheat
[21:41:51.122] Vile Swill's Rend is refreshed by Thecheat
[21:41:51.122] Vile Swill's Rend is refreshed by Thecheat
[21:41:51.122] Vile Swill's Rend is refreshed by Thecheat
[21:41:51.122] Vile Swill's Rend is refreshed by Thecheat
Notice only 4 targets take damage (the 5th was a miss), but 5 Vile Swills still get Rend refreshed regardless.
Tier 12 Example (Source:
World of Logs - Real Time Raid Analysis)
[20:24:10.443] Thecheat casts Thunder Clap
[20:24:10.443] Thecheat Thunder Clap Ragnaros Miss
[20:24:11.113] Ragnaros's Rend is refreshed by Thecheat
Only one target in this scenario, but that consequently makes this much easier to see. Rend refreshes as expected.
Tier 13 Example (Source:
]World of Logs - Real Time Raid Analysis

[20:22:03.679] Thecheat casts Thunder Clap
[20:22:03.679] Thecheat Thunder Clap Forgotten One Miss
[20:22:03.679] Thecheat Thunder Clap Forgotten One Miss
[20:22:03.893] Thecheat Thunder Clap Yor'sahj the Unsleeping 5792
[20:22:03.893] Thecheat Thunder Clap Forgotten One 5401
[20:22:03.893] Thecheat Thunder Clap Forgotten One *10800*
[20:22:03.893] Thecheat Thunder Clap Forgotten One 5468
[20:22:03.893] Thecheat Thunder Clap Forgotten One 5400
[20:22:03.907] Thecheat Thunder Clap Forgotten One 5400
[20:22:03.907] Thecheat Thunder Clap Forgotten One 5400
[20:22:03.907] Thecheat Thunder Clap Forgotten One 5400
[20:22:03.907] Thecheat Thunder Clap Mana Void 5552
[20:22:03.907] Thecheat Thunder Clap Forgotten One 5401
[20:22:03.907] Thecheat Thunder Clap Forgotten One 5401
[20:22:04.219] Forgotten One afflicted by Rend from Thecheat
[20:22:04.219] Forgotten One afflicted by Rend from Thecheat
[20:22:04.219] Mana Void afflicted by Rend from Thecheat
[20:22:04.219] Forgotten One afflicted by Rend from Thecheat
[20:22:04.219] Forgotten One afflicted by Rend from Thecheat
[20:22:04.219] Forgotten One afflicted by Rend from Thecheat
[20:22:04.219] Forgotten One afflicted by Rend from Thecheat
[20:22:04.219] Forgotten One afflicted by Rend from Thecheat
[20:22:04.236] Forgotten One afflicted by Rend from Thecheat
[20:22:04.236] Forgotten One afflicted by Rend from Thecheat
[20:22:04.236] Yor'sahj the Unsleeping's Rend is refreshed by Thecheat
Heavy AoE situation, but still easy to see behavior. Thunder Clap targets 13, hits 11, and only 11 get/refresh Rend. This behavior is changed from the way it has worked the entire expansion, thus leading me to conclude it's bugged. Come to think of it, I should probably bump my bug report post and include these logs.